RF Emissions and the 702, 722, and 744T Recorders

Note: While this article mentions Lectrosonics products specifically, the facts and solutions apply generally to most professional wireless receivers.

Sound Devices 702, 722, and 744T hard disk recorders have broken new ground for size and performance of modern digital audio recorders. As with the introduction of many new product categories, however, some things that were taken for granted in the past may have to be re-thought moving forward. One area where the modern recorder is different than that of earlier generations is in its use of computing power.

The RF (radio frequency) emissions of hard-disk-based digital recorders are higher than that of the tape-based recorders they replace. This increased stray RF energy can cause interference with wireless receivers in close proximity (within the same bag), reducing the useable range of the wireless microphone system. Some customers experience no performance issues, while others need to take corrective action. With careful planning, however, the 702, 722, and 744T recorders can co-exist with wireless systems with excellent results.

7-Series Architecture

The 702, 722, and 744T digital recorders can be thought of as high-powered, application-specific audio computers. Similar to notebook computers, desktop computers, digital recorders, and other digital devices, they radiate RF (radio frequency) energy, sometimes in significant amounts. These recorders contain numerous digital circuits, including:

  • high-powered digital signal processors,
  • hard disk storage (some models) and CompactFlash storage with associated circuitry
  • high-speed switching power supply for convenient, flexible powering options
  • high-accuracy time code circuitry based on a temperature-compensating crystal oscillator (some models) and high accuracy word clock generating oscillators.

Because of the nature of high-speed digital switching, all of these required elements inherently generate RF energy and they do so at different levels and at different frequencies. While the metal chassis of the recorders absorb a certain amount of energy, it cannot contain it all. RF emissions and susceptibility are evaluated, designed, and tested in Sound Devices product development stage. Additionally, Sound Devices has experimented with added sheilding in areas, such as the rear battery holder with little change in their RF emission.

It is important to note that every shipping digital recorder on the market, whether it is a $500 unit with a plastic chassis or a $20,000 powder-coated showpiece, emit considerable RF energy in the critical 500 MHz to 800 MHz regions. The present generation of digital recorders are just the beginning of a wave of high-powered digital devices used in recordists’ bags. The enormous benefit of digital technology won’t be slowed by their potential to cause interference.

Wireless Receivers

Modern wireless microphone receivers used in production sound are incredible works of technology. In general, these receivers are very sensitive and can receive RF signals from low powered transmitters while still achieving excellent usable range. For example, the Lectrosonics 411 receivers have a rated sensitivity better than 1 uV, making it capable of picking up very low level radio signals. The signal scanning utility on a Lectrosonics 411 receiver indicates signal present at 1 uV (-107 dBm), with each of the 11 remaining pixels showing a 5 dB increase in signal.

Problems can arise when high-sensitivity wireless receivers, such as Lectrosonics are used in the same bag as digital devices, such as Sound Devices 702, 722, and 744T recorders. Much like an air conditioning vent will cause acoustic noise pickup when next to a sensitive microphone, a wireless receiver picks up RF energy from the nearby digital devices in addition to the wanted transmitter source.

There are two coupling mechanisms: radiated RF and conducted RF. Conducted RF coupling can arise when wireless microphones are powered from the same power supply as the digital device; the RF energy travels through the power supply cables. Radiated coupling occurs through the air, from the recorder to the receiver’s antennas.

Quick Tips to Improve RF Performance

There are several “best practice” steps which can be taken to reduce the likelihood of interference.

  1. Ground the shielding wire on audio cables connected to the receiver. Adding a short wire inside the XLR input connector connecting the shell to the shield-pin substantially reduces the RF emissions, significantly increasing wireless range. This tech note details the procedure.
  2. Avoid wireless frequencies where the recorder emits high RF energy. Included in this tech note are graphs of the recorder’s RF output emissions (at 48 kHz). Note: Because Lectrosonics receivers are a popular companion to Sound Devices recorders in North America, the graphs below are broken into Lectro’s system of “blocks”, each containing 256 usable frequencies. Take advantage of wireless manufacturers websites for frequency coordination as a start, instead of trial and error in the field.
  3. Make certain that RF is not being conducted through power cables. The 702, 722, and 744T recorders emit low levels of RF energy through the power cable. One way to determine if this is causing interference problems is to power the recorder and wireless from their own individual power sources and note any change in performance. Power problems can be remedied by battery powering either the recorder or receivers. If a single power source is powering recorders and receivers wrap several turns of the recorder’s power cord through a ferrite toroid and wrap the power cord for the receivers through their own toroids.
  4. Keep as much physical distance between the receiver’s antennas and recorders as possible. RF noise reception falls as the square of the distance (and in the near field, as the cube of the distance). Increasing the distance by even one inch when in close proximity helps. Increasing distance from one inch to one foot can improve a problem situation dramatically. While this separation has not been necessary in the past with a bag filled with analog gear, if any manufacturer’s digital recorder is used in a bag, interference can become an issue, at certain frequencies. Just like in the analogy with the sensitive microphone, the best cure is to move the microphone away from the noise source. The same is true of the receiver’s antenna. Wireless manufacturers like Lectrosonics are exploring shoulder harness mounted antennas.
  5. Placement of metal shielding material between the recorder and receiver can provide minor improvement from experience, although this may be attributed to repositioning the antennas relative to the recorder rather than the effects of the material. In the very near field (really close to the recorder chassis), the RF field pickup changes dramatically with slight repositioning. The better solution is remote mounting of the antennas.

Emission Graphs

744T Block 21

744T Block 22

744T block 22

744T Block 23

744T block 23

744T Block 24

744T block 24

744T Block 25

744T block 25

744T Block 26

744T block 26

744T Block 27

744T block 27

744T Block 28

744T block 28

744T Block 29

744T block 29

744T Block 30

744T block 30

744T Block 31

744T block 31

744T Whip Antenna Block 21-26

744T whip 21_26

744T Whip Antenna Block 27-31

744T whip 27_31

Wrap it Up

With thoughtful planning the 7-series recorders can happily co-exist with wireless systems with excellent results. However, the happy days of finding a clear frequency block in a particular geographic region has changed with the proliferation of digital television transmission and now digital recorders.

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